In virtually all fields of communications, there exists a persistent demand to transmit more data in less time. The amount of information that can be transmitted over a communications system (or through a component of that system) is referred to as the bit rate or the data throughput of the system. Traditionally, system throughput is increased by either increasing the number of channels carrying information or increasing the bit rate of each channel. In order to meet ever-increasing bandwidth demands, aggregate throughput in fiber optic transmission systems has conventionally been increased by using multiple Wavelength Division Multiplexed (WDM) channels, time-division-multiplexing (TDM), or some combination of the two techniques. WDM techniques increase the number of channels transmitted on a particular fiber, while TDM techniques increase the data rate of each individual channel.
Conventional optical fiber networks typically can deliver on the order of 10 Gigabits of data per second (10 Gb/s). Both WDM and TDM techniques have been applied to realize fiber channel bit rates well above this conventional 10 Gb/s capacity. Many fiber optic communication systems comprise multiple WDM channels simultaneously transmitted through a single optical fiber. Each of these channels operates independently at a given bit rate, B. Thus for an m channel WDM system, the system throughput is equal to m·B. Conventional dense WDM (DWDM) systems typically operate with 40 to 100 channels. There are certain restrictions, however, that limit the aggregate power that can be transmitted through a single DWDM optical fiber (i.e., the launch power). For example, eye safety power regulations and nonlinear effects in the fiber place limits on the aggregate launch power. In addition, channel spacing limitations and per-channel launch power, effectively limit the number of WDM channels that can be combined for transmission on a single fiber.
TDM techniques also are associated with various limitations. For example, using conventional TDM techniques to achieve an n-times increase in channel data rates requires the optical components of a link (e.g., the modulator and photodetector) to be replaced with new optical components having n-times the bandwidth of the original optical components. In addition, the interface circuitry must be replaced with new circuitry having bandwidth n-times greater than the original circuits.
Optical fiber networks are typically comprised of a series of links that include a transmission block, a receiver block, and a long stretch of optical fiber connecting the two blocks (i.e., the optical plant). FIG. 1 is a block diagram of a conventional m-channel WDM fiber optic transmission system link 100. The fiber optic transmission system link 100 consists of a WDM transmission block 102 (denoted as the “Head”), the optical fiber 104, and a WDM reception block 106 (denoted as the “Terminal”). The Head 102 comprises m transmitters 108–112 (labeled “Tx”) and an m-channel WDM multiplexer 114. Each transmitter 108–112 comprises an optical source (not shown) and all circuitry necessary to modulate the source with the incoming data stream. For the case of external modulation, the transmitter block also includes a modulator. The Terminal 106 comprises an m-channel WDM demultiplexer 116 and m receivers 118–122 (labeled “Rx”). Each receiver 118–122 comprises a photodetector (not shown) and all circuitry required to operate the detector and amplify the detected signal in order to output the original electrical data stream.
In order to realize channel data rates of 10 Gb/s and beyond, the optical fiber 104 as well as the Head 102 and Terminal 106 of the link 100 are typically upgraded to support the increased data rates. In order to increase the channel bit rates in this conventional link 100, each transmission block 102 and reception block 106 must be replaced with optical components and circuitry capable of achieving the desired bandwidths. For high-speed channel bit rates (10 Gb/s and faster), the optical fiber 104 also must often be replaced in order to compensate for signal distortions, which are more prominent at higher data rates. This process can be particularly cumbersome and costly in a long-haul link where hundreds of kilometers of fiber must be replaced. For existing long-haul optical links, the complexity and cost of replacing planted fiber often represents a prohibitive barrier for increasing channel bit rates.
Service providers seeking to optimize revenue and contain cost prefer a highly granular, incremental expansion capability that is cost effective while retaining network scalability. The ability to increase the throughput capacity of single point-to-point links or multi-span links without upgrading or otherwise impacting the remainder of the network is highly desirable from an engineering, administrative and profitability standpoint.
In view of the foregoing, there is a need for a method of increasing a channel data rate in a fiber optics communication link that does not require replacing an existing optical fiber plant or necessitate a change in the expensive optical components. There exists a further need to increase the efficiency of the available spectrum within a given fiber optic communication link and to obtain efficient highly granular bandwidth upgrades without upgrades to an existing optical fiber plant, upgrades to channel combing optics, or significant changes to existing maintenance and administrative procedures. The method should further allow service providers to increase data throughput on a per-link basis as throughput demands increase, generating higher profitability for the service provider and lower cost for the consumer.